Industrial process transmitters are devices that can be coupled to industrial process equipment and/or conduits and are adapted to measure process parameters, such as pressure, mass flow, flow rate, temperature, and the like. Frequently, such transmitters draw power from a two-wire loop that carries an energy limited loop current, which varies within a range of 4-20 mA. When the current is low (such as 4 mA), a majority of the power available to the transmitter from the loop is used by circuitry within the transmitter to sense a process variable and to generate a process variable output representative of the sensed process variable.
In some configurations, transmitters can utilize primary and secondary process measurements, using multiple sensors or field devices. For instance, to make a mass flow measurement of gas or steam through a pipe, a flowmeter can be used to measure flow rate, and a second sensor can be used to measure the line pressure, for example.
Power delivery to the sensor or field device performing such secondary process measurements contributes to the overall current and power consumption of the system. At low current levels (such as 4 mA), very little power (typically 1 to 2 milliwatts) is available for powering accessory loads and for communicating with feature modules.